Human SLC-1 found in human genome (FEBS Letters 398 (1996) 253-258) and rat SLC-1 found from a rat brain cDNA library (Biochimica et Biophysica Acta, 1401 (1998) 216-220) are collectively referred to as a G protein-coupled receptor or seven transmembrane receptor. They are one of many G protein-coupled receptor proteins called orphan receptors, because their ligands are unknown.
For determining ligands of these orphan G/protein-coupled receptor proteins, there was no other general means but only to predict the ligands from similarity in the primary structures of these G/protein-coupled receptor proteins. However, many orphan G protein-coupled receptor proteins are poor in homology to known receptors. Actually, it was difficult to predict the ligands only from the similarity in their primary structures, except that they are receptor subtypes of known ligands. On the other hand, since many orphan G protein-coupled receptor proteins were found by gene analysis, it is predicted that there will be still many other unknown ligands corresponding to those proteins. To date, however, only few ligands were identified for these organ G protein-coupled receptors. With respect to SLC-1, it has not been reported, either, if its ligand exists.
To survey ligands to the SLC-1 orphan receptor protein and establish a method for screening a compound, etc. characterized by using SLC-1 and its ligands have thus been the problems to be solved.